A reception waiting time of a transportable wireless device is preferably extended as long as possible, by reducing power consumption and extending battery life. In a wireless communication, while signals are not being received and a wireless communication device is waiting for arrival of a signal (during a reception waiting time), functions of a receiver in the wireless communication device may be limited to a minimum, by only using the receiver for determining a received level of a reception signal and the like. For example, a circuit, a digital signal processor (DSP), or the like that performs a receiving process is operated in a power saving mode. More specifically, when a field intensity of a reception signal, in other words, a received level is less than a threshold, power consumption of the device may be reduced by lowering a speed of an operation clock of a processor in the device than that when the received level is high.
Japanese Laid-open Patent Publication No. 2012-34287 discloses a technology in which demodulation is started when a detected received level is equal to or more than a predetermined threshold. In Japanese Laid-open Patent Publication No. 2012-34287, a received level detector intermittently performs a signal level determining process at a reception cycle, to reduce power consumption of a wireless communication device. When the signal level is equal to or more than a predetermined threshold, a receiving circuit starts the demodulation.
In a digital wireless system, data is transmitted and received in a signal format and frame structure that are defined in advance in a specification of the systems. When data is transmitted and received in the signal format and the frame structure, a synchronization frame is basically inserted in each frame period. Consequently, the receiving device extracts a synchronization word from the received signal and specifies the frame.
When the receiving device intermittently determines arrival of a reception signal in a fixed cycle for a power saving operation and the like to thereby detect the arrival of the reception signal, the arrival of a signal is first detected by intermittently determining the arrival of the reception signal, and then the reception signal is demodulated. During this process, when a timing at which the arrival of the reception signal is determined is deviated from a timing at which the reception signal starts arriving, a synchronization frame just after the reception signal starts arriving cannot be detected. Thus the frame cannot be confirmed, thereby omission of a head part of the reception signal occurs and demodulation of the reception signal cannot be performed. In particular, in a receiving device used in an emergency such as in a police wireless system or a fire wireless system, the omission of the head part of the reception signal is not preferable.